Short definition
A combined sewer carries sanitary waste and storm runoff in a single pipe. It’s the legacy default in pre-WWII US cities and is still in service across roughly a third of Seattle. In big rainstorms, a combined sewer can surcharge — pushing sewage back up through low-lying basement drains and, at city outfalls, releasing untreated discharge to the receiving waterway. Backwater valves are the standard homeowner protection.
What it is
In a separated drainage system, the sanitary sewer carries household waste to a treatment plant and the storm sewer carries roof, street, and surface runoff to a creek, lake, or the Sound. In a combined system, both flows share one pipe. That works fine in dry weather (waste alone) but becomes the system’s weak point under heavy rainfall, when storm volume can exceed waste volume by 50× or more.
When capacity runs out, two things happen. First, the combined main backs up, and the lowest basement floor drain in any home connected to it can become an inlet. Second, at engineered outfalls along the shoreline, combined sewer overflows (CSOs) discharge a mix of stormwater and untreated sewage to receiving waters to prevent the system from rupturing.
Why it matters to a homeowner
If you live in a Seattle combined-sewer neighborhood — Ballard, Fremont, Wallingford, the U-District, Capitol Hill, the Central District, downtown, parts of West Seattle and Beacon Hill — heavy rain is a basement-flood risk in any home with floor drains, basement bathrooms, or laundry hookups below street level. The standard protection is a backwater valve on the building drain or side sewer, which lets sewage flow out but slams shut against backflow.
If the city sends a notice that a “combined sewer overflow occurred near your address,” that means an outfall briefly discharged to the waterway during a storm — it doesn’t mean your home was flooded, but it’s a flag that the local main was at capacity. Permits for new basement bathrooms in CSO areas often require backwater-valve protection as a condition of approval.
When you’ll encounter this term
- Basement floods in heavy rain — a plumber asks if you’re in a combined-sewer area.
- City notice of a CSO event near your address.
- Adding a basement bathroom in Seattle and the plumber requires a backwater valve.
- Reading about Seattle’s Overflow Action Plan or the SPU/King County Consent Decree.
Common variants / not the same as
- Combined sewer vs. sanitary sewer. Sanitary carries waste only; combined carries waste + storm.
- Combined sewer vs. storm drain. Storm drains carry runoff only and discharge to surface waters; combined sewers carry both and (mostly) flow to the treatment plant.
- Combined sewer overflow (CSO) vs. sanitary sewer overflow (SSO). A CSO is by-design at an engineered outfall; an SSO is an unintended sanitary release and is regulated more strictly.
Washington note
Roughly one-third of Seattle is in a combined-sewer service area; the rest of the city is separated. Seattle Public Utilities manages about 82 smaller CSOs and King County Wastewater Treatment manages 38 larger outfalls. The Seattle Overflow Action Plan covers planning through 2026 and design and implementation 2026–2037 under a modified Consent Decree (May 22, 2025). Tacoma also has legacy combined sewers in the older downtown and waterfront core. Bellevue, Redmond, Kirkland, and most newer suburbs are fully separated and not affected.
If you live in an affected neighborhood, a working backwater valve and a verified main-cleanout location are the two upgrades worth more than they cost.